The invention relates to micro-electromechanical system (MEMS) devices, and, more particularly, to devices including MEMs tunable capacitors and related methods.
Many common communications devices include components, such as phase shifters, resonators, filters, etc. which, in turn, include one or more tunable capacitors. The capacitance value of the tunable capacitor can be selectively varied to provide a desired capacitance in the circuit. The capacitance can typically be selected by varying the spacing or overlap between adjacent capacitor electrodes, and/or by changing the dielectric material between the electrodes.
Certain applications have developed wherein relatively small tunable capacitors are desirably used. Such capacitors can be made using MEMS manufacturing processes wherein very small movable components can be formed on a substrate using a combination of deposition, plating or other additive processes, and selective etching, and/or other lift-off techniques. Such techniques typically form a structure which is ultimately partially released or suspended to permit mechanical motion, typically as a result of an electrostatic force. The electrostatic force may be generated by applying an electrical voltage to spaced apart conductors. One common MEMS structure is a switch provided by a conductive beam anchored at one end and with an opposite end that can be brought into engagement with an adjacent contact via an applied electrostatic force.
One approach to providing a selectable circuit capacitance is disclosed in U.S. Pat. No. 5,880,921 to Tham et al. The patent discloses a plurality of capacitors connectable in parallel using MEMS switches to thereby provide a wide range of digitally controllable capacitance values. U.S. Pat. No. 5,808,527 to De Los Santos et al. discloses a similar use of MEMS switches to configure an array of capacitors to provide a desired capacitance value.
An article entitled xe2x80x9cDistributed MEMS True-Time Delay Phase Shifters and Wide-Band Switchesxe2x80x9d by Barker et al. appearing in IEEE Transactions on Microwave Theory and Techniques, Vol. 46, No. 11, November 1998, at pp. 1881-1890, discloses a MEMS bridge having a capacitance value which is variable in an analog fashion based upon an applied pull-down voltage. More particularly, the bridge is anchored at opposing ends, with a conductive layer under the bridge defining one plate of the capacitor. The conductive layer is coated with an insulating layer of silicon nitride to prevent shorting should the bridge be brought fully downward.
Unfortunately, contact between the bridge and the underlying contact of the Barker et al. MEMS capacitor can frequently cause sticking, or xe2x80x9cstictionxe2x80x9d and thus render the device unusable. In addition, analog control of the position and thus capacitance can be very complicated, as individual capacitors may vary in a given device, and individual capacitors may also vary over time and/or temperature.
In view of the foregoing background, it is therefore an object of the invention to provide an electronic device including a MEMS capacitor and associated manufacturing method to produce the capacitor having selectable values and which is low cost, reliable, and readily controlled.
This and other objects, features and advantages in accordance with the present invention are provided by an electronic device comprising a substrate, and a MEMS capacitor on the substrate and having a plurality of selectable capacitance values. More particularly, the MEMS capacitor preferably includes a lower capacitor electrode on the substrate, and a movable bridge comprising end portions connected to the substrate laterally adjacent the lower capacitor electrode. The movable bridge may also include a conductive medial portion between the end portions defining an upper capacitor electrode suspended above the lower capacitor electrode. The upper capacitor electrode is thus movable between an upper position and a lower position by an electrostatic attraction force generated between the capacitor electrodes by an applied voltage. The upper and lower positions provide respective low and high selectable capacitance values. Moreover, the movable bridge may further include at least one travel limiting portion between the end portions for engaging adjacent substrate portions to keep the upper capacitor electrode in a predetermined spaced relation from the lower capacitor electrode when in the lower position. This travel limiting feature of the MEMS capacitor is relatively easy to fabricate and avoids the sticking or stiction problem of some other types of MEMS capacitors.
In some embodiments, the at least one travel limiting portion may comprise first and second travel limiting portions on opposite sides of the upper electrode or the medial portion of the movable bridge. The movable bridge may also have a shape defining a plurality of vertical steps. These vertical steps may descend from the end portions to respective travel limiting portions and ascend from respective travel limiting portions to the medial portion of the movable bridge. In other words, the movable bridge may have a stair-step construction, and wherein lowermost steps or portions define the travel limiting portions.
Another aspect of the invention is that the lower capacitor electrode may comprise a conductive uppermost surface portion so that an air gap provides a dielectric for the MEMS capacitor in both the upper and lower positions. In other words, a dielectric layer or coating need not be fabricated on the upper surface of the lower capacitor electrode.
The electronic device may further include a controller for applying control voltages to the lower and upper capacitor electrodes of the MEMS capacitor to select between discrete low and high capacitance values defined by the upper and lower positions, respectively. The controller, for example, could provide no voltage for the upper position and corresponding low capacitance value. For the lower position and corresponding high capacitance value, the controller could provide a voltage which exceeds a predetermined threshold voltage to provide the electrostatic force to bring the movable bridge down to the lower position until the travel limiting portions are engaged.
The movable bridge may be an integrally formed monolithic structure and may have an elongated strip shape. In addition, the movable bridge may have a configuration imparting an upward bias thereto. Accordingly, upon discontinuing the control or pull-down voltage when in the lower position, the capacitor will return to the upper position.
The MEMS capacitor may include a reinforcing layer on the medial portion of the movable bridge. In some embodiments, the substrate may comprise an insulating material. The lower capacitor electrode and the movable bridge may each comprise metal, such as gold, for example.
A method aspect in accordance with the invention is for making an electronic device including the MEMS capacitor as described above. The method may include forming a lower capacitor electrode on the substrate, and forming a movable bridge comprising end portions connected to the substrate laterally adjacent the lower capacitor electrode. A conductive medial portion is formed between the end portions defining the upper capacitor electrode suspended above the lower capacitor electrode. This upper capacitor electrode is movable between an upper position and a lower position by an electrostatic force generated between the lower and upper capacitor electrodes. The upper and lower positions provide respective low and high selectable capacitance values for the device.
Of course, forming the movable bridge may further comprise forming at least one travel limiting portion between the end portions for engaging adjacent substrate portions to keep the upper capacitor electrode in a predetermined spaced relation from the lower capacitor electrode when in the lower position. In addition, forming the lower capacitor electrode may comprise forming the lower capacitor electrode to have a conductive uppermost surface portion so that an air gap provides a dielectric for the MEMS capacitor in both the upper and lower positions.